Weather conditions produce a variety of frozen precipitation: ice, snow, sleet, hail, frost, and slush. The problems created by such precipitation when it covers surfaces such as airplanes, runways, streets, roads, parking lots and sidewalks are well known. In general terms, the solution is simply to remove the ice and/or to prevent it from reforming or re-adhering to the surface.
The problems associated with ice formation on the surfaces of aircraft are particularly acute, both because the demands placed on a deicing composition are severe and because the consequences of ice formation can be catastrophic. Thus, this invention, although it may be applied to other systems, shall be discussed generally in the context of that most demanding system. Unless indicated otherwise, when this specification refers to ice it shall be understood that the term encompasses all forms of frozen water by whatever name they may be known.
The art seemingly is complete, and it evidences a number of strategies for preventing the formation and reformation of ice. The most obvious strategy is to depress the melting point of water by admixing, or by allowing the admixture of other materials. For example, in U.S. Pat. No. 2,200,184 to Morgan, it is disclosed that large proportions of alkanol amines effectively depress the freezing point of water into which it is added. Perhaps the most common antifreeze and deicing compositions, however, are based on ethylene glycol and other alkylene polyols.
German Pat. No. 3,142,059 to Schwenk discloses aircraft deicing fluids based on alkylene gylcols having 2 to 3 carbon atoms and oxalkylene glycols having 4 to 6 carbon atoms, and preferably on ethylene and propylene glycol. U.S. Pat. No. 4,358,389 to Konig-Lumer discloses very similar aircraft deicing fluids based on the same glycols, and cites and discusses other such patents, e.g., U.S. Pat. No. 2,373,727, German Pat. No. 1,901,061, and German Pat. No. 2,423,893.
Simply depressing the freezing point of water, however, does not provide a complete solution. Other strategies have been adopted. In an entirely different context, certain additives were utilized to enhance the effectiveness of ethylene glycol by reducing the mechanical strength of ice.
In U.S. Pat. No. 4,254,166 to Glanville, the problem addressed was the freezing of particulate material, such as coal, which may be moved and stored under subfreezing conditions. By adding certain highly soluble inorganic or organic compounds containing an ammonium ion, it was discovered that the mechanical strength of ice was greatly reduced. Glanville also disclosed that for economic reasons, it may be desirable to add various surface active agents, such as anionic surfactants, and particularly, non-ionic surfactants of the polyethoxylated type. Such compounds also reduce the mechanical strength of ice, presumably by interfering in some way with the crystalline structure.
Another study focuses on the thermal exchange properties of ice/water systems. In U.S. Pat. No. 3,412,030 to Wahlberg, a method for accelerating the melting of snow was disclosed. It consisted, in essence, of dissolving in situ anionic, non-ionic, or cationic wetting agents. A wide variety of wetting agents, including alkylaryl sulfonates, is disclosed as useful. It should be noted, however, that non-ionics, e.g., ethoxylated alcohols, tend to oxidize easily and are explosive.
Although Wahlberg acknowledged that it had not been fully determined, he suggested a partial explanation. By dissolving wetting agents therein, the water film formed upon initial thawing has a lower surface tension, and water more readily runs off the ice surface or into the snow. The thermal exchange between the ice and air is enhanced in two ways. Firstly, the ice is placed thereby in more direct contact with the surrounding air and/or warmer air is permitted to flow into the interior regions of the snow cover. Secondly, there is less water on the surface which might otherwise absorb surrounding heat, evaporate, and further cool remaining water and the ice surface.
Preventing the reformation of ice is a related, but in many ways, different problem. It is well known that surfactants may be used to create on aircraft a temporary or semipermanent surface coating having a low adhesion for ice. See A. Schwartz, et al., 2 SURFACE ACTIVE AGENTS AND DETERGENTS 398-408, 729-30 (Interscience Pub., Inc., New York 1958). Among the surfactants used for that purpose are the sorbitol esters of fatty acids, the lower alkylnapthalenesulfonates, and the silicones.
It also is known that among other water-soluble surfactants, alkylaryl sulfonates are useful as anti-fogging agents. The mechanism is similar: a water repellent film is laid down on the protected surface. Generally, those surfactants are used in glycol bases with spreading and antifreeze agents. Anionic sulfonates, when used in conjunction with polyglycol esters of fatty acids, are especially suited for plastic surfaces. Long-chain cationic surfactants also are known to lay down a water-repellent film on glass surfaces, even when applied via dissolution in an aqueous solution.
The demands on aircraft deicing fluids, however, are particularly acute. Their properties may be discussed as three general and related sets of properties: functional, compatibility, and application.
The functional requirements, in a sense, are the most obvious. The fluid must remove existing ice from the aircraft surface and prevent ice reformation for at least eight hours. Of course, they themselves must not freeze at the application temperatures.
The compatibility requirements are many, because the typical aircraft encompasses a variety of surfaces: aluminum, titanium, magnesium, alloys thereof, high-strength steel, glass, acrylic, paint, decals, and electrical connections. Deicing fluids should not leave deposits, corrode or embrittle metal, soften or bubble paint and decals, or craze acrylic. Their flash point should be sufficiently high so that the plane's electrical system does not ignite the fluid.
Not only must they be compatible with all aircraft surfaces, but their viscosity, specific gravity, stability and other flow and shear properties must allow for proper application by conventional spraying techniques. All surfaces should be covered. The fluid should sheet and remain on the surface long enough to deice and protect the aircraft, but excess quantities should not interfere with the aerodynamic qualities of the plane.
The properties of acceptable deicing fluids, and the testing procedures for determining those properties are outlined in Publications AMS 1425A and AMS 1427, revised/issued on Apr. 1, 1982, and Oct. 1, 1981, respectively, by The Society of Automotive Engineers, Inc., 400 Commonwealth Drive, Warrendale, Pa. 15096 (respectively, for ethylene glycol and propylene glycol based fluids). Those publications are herein incorporated by reference.
Despite the apparent completeness of the prior art in this field, to date the inventor herein is unaware of any fluid which is able to meet those specifications in their entirety. Generally, the prior art deicing fluids have had problems in maintaining long lasting protection against ice reformation without compromising the compatibility properties of the fluid. Specifically, ethylene glycol alone does an excellent job of removing existing ice and is entirely compatible with the aircraft. It does not provide protection against reicing, however, except in the very short term. Additives designed to provide long term protection against reicing generally compromise the fluid's compatibility, particularly in that decreased flash points, acrylic crazing, paint bubbling, and metal hydrogen embrittlement often are observed.